are there any difference between ndk-bundle obtained donloading the ndk from SDK manager and android-ndk-r15c from android developers? I'm not able to compile a project cloned from git-hub, in the description is write that I need android-ndk-r8c... I'm using ndk-bundle, is the same??
Yes, ndk-bundle directory contains exactly same NDK as what you can download from https://developer.android.com/ndk/downloads/index.html. But if the project is tuned for r8c, you may have some work to do to tune it for r15c.
This does not mean that you should download r8c: some important bugs have been fixed, and there may be compatibility issues with new devices.